Cross talk eliminator



Aug. 18, 1936. J. EVANS CROSS TALK EL IMINATOR Filed Sept. 2'7, 1934 VIE A INVEN'I'OR John Evans HT'T'ORNEY Patented Aug. 18, 1936 UNITED STATES PATEN oFFicE 4 Claims. (Cl. 250-47) This invention relates to apparatus for crosstalk elimination when used in connection with radio communication systems.

The device has particular utility in systems operated at ultra high frequencies. My cross-talk eliminator may be employed either at a radio transmitter or adjacent a receiver. In either case it is usually necessaryto provide an electrical circuit of appreciable length between the antenna and the radio apparatus. If two or more ultra high frequency transmitters are located in close proximity to one another and their radiating frequencies happen to be sufficiently close so as to produce undesirable beats, due to rectification of energy in the platecircuits, it then becomes a problem how to avoid the deleterious effects of such mutual interference.

Accordingly, it is among the objects of my invention to provide a cross-talk eliminator which is adapted to be connected to the transmission lines ordinarily employed between an antenna and a radio apparatus such that interfering Waves of a frequency other than the operating frequency of the radio apparatus will be substantially drained to ground or s'hort-circuited before they can have any deleterious effect.

Another object of my invention is to provide a cross-talk eliminator having manually adjustable means for the introduction across the transmission lines to an antenna system of a low impedance path with respect to interfering waves of frequencies lower than the operating frequency of the system.

Another object of my invention is to provide adjustable means suitably shielded whereby two conductors of a transmission system may be interconnected, the interconnecting path being of substantially a half-wave length at the operating frequency of the system.

The foregoing and other objects and advantages of my invention will best be understood upon reference to the following detailed description in connection with the accompanying drawing, in which Figure 1 shows partly diagrammatically and partly in cross section one embodiment of my cross-talk eliminator; and

Fig. 2 shows a sectional view of the device, the section being taken in the plane 2--2 of Fig. 1.

It is well known that when some definite frequency is being impressed upon a given two-wire transmission line, two points on said line may be selected which are electrically degrees out of phase with one another and these points may be interconnected by a bridging conductor which,

when adjusted to ahalf-wave length, offers practically an-infinite impedance to alternating currents of said frequency, provided certain precautions are, taken to minimize radiation losses. For stray frequencies other than the operating frequency inthe transmission line, however, such a-bridgingconductor affords a very low impedance. It may readily be seen, therefore, that if I provide adjustable means for tuning the bridging connection between the two conductors of a transmission line, that I am-able to suppress substantially all parasitic. frequencies lower than the working frequency of the system.

Referring to Fig. 1, I show an antenna 3 and counterpoise or ground 4, to which are respectively connected the transmission wires 5. At the other ends of these wires-I may connect the Winding 6 of a transformer 7. The other winding 8 of this transformer may, if desired, be made the primaryupon which is impressed electrical energy from any suitable source 9. The winding 8 preferably, but not necessarily, has a center tap H] which is. grounded. Along the transmission lines 5 suitable points H may be selected at which to connect my cross-talk eliminator. These points I I may be selected at any convenient part of the transmission line, provided they are substantially side by side so as to have opposite polarity.

Connecting with the transmission lines at each of the points II is a rod-like conductor l2 substantially surrounded by and coaxiallydisposed Withrespect to the shielding members 13. The length of these conductors and cylindrically shaped shields is determined by the lowest frequency atwhich the radio and antenna system 3 are to be operated. With the structure as shown it is essential that a path be provided through the conductors 'l2 and through a bridging member 1'4 interconnecting the same such that the path will have an electrical length of substantially a half-wave length between the points H on the transmission lines 5.

Within certain tolerances it appears to be desirable, in order to minimize the losses, that the outside diameter of the conductors l2 and the inside diameter of the cylindrical shields l3 bear a certain ratio to one another. This ratio is preferably of the order of 113.6. 7

The conductors l2 and shielding cylinders I3 may be mounted on and supported by any suitable non-ferrous metallic platform structure such as the member l5. Sockets l6 may be provided, one at the bottom of each conductor l2, for centering the same within the cylinder. For a like purpose an insulating member I1 may be ,way of illustration, as constituted by a threaded;

shaft l8 engaging with an internally threaded portion of the bridging member I4. At oneend of the shaft I8 I preferably employ a bevel 'gearl9'which meshes with a driving gear 20 fixedly mounted on a shaft 2| which may be rotated in any suitable manner, as by means of a crank 22. The shaft l8 may be journaled at the upper end in a member which spans the space he tweenfthejtwo cylinders 13. The shaft l8 may also be journalled at the lower end as by a hole 24 in the base IS. A clearance hole is made in the cross-arm 23 to'accommodate the shaft I8.

The conductive bridge I4 is provided with suitable resilient brushes 25 at each end whereby positive contact with the rods l2 may be assured. Slots 26 are provided in the cylindrical shields I3 so that the bridging member l4 may move up and down therein for contacting with the conductor bars l2 at any desired level. The slots in the cylinders also serve to restrain the bridging member M from turning when the threaded shaft I8 is rotated. 1

If my cross-talk eliminator is to be used in connection with a 'radio'transmitterfit will be seen that oscillations of any desired carrier frequency'may be generated at thesource 9 and transmitted first through the transformer I, and

'for frequencies lower than the operating frequency and at the same time offer a practically ,infinite impedance to. currents of the operating frequency.

If my cross-talk eliminator is to be used in connection with a radio receiver its application and utility will readily be recognized in view of the foregoing description. In this case, however,

the antenna; and counterpoise 4 will be viewed as the source of energy and the radio receiver will be assumed to occupy the position 9.

While I have shown but one embodiment of my invention by way of illustration, it will be understood by those skilled in the art that many modifications may be made without departing from the spirit and scope of the invention itself.

being substantially of the order of one to three.

and six tenths,:a conductive bridge disposed in contact with said conductors, said bridge having.

aninternally threaded portion, and means including an adjusting screw engaging with said internally threaded portion for varying the position of contact of said bridge across said conductors whereby the conductors and bridge are caused to provide a path of high impedance to electric waves of the operating frequency impressedupon said transmission system when said interference eliminator is connected across points of half-wave phasexdifference in said system.

2. In an ultra high frequency system, an antenna and counterpoise, radioapparatus, adjacently disposed conductors interconnecting said antenna and counterpoisewith said apparatus,

and an interference eliminating device connected to said conductors at points of substantially degrees phase difference at the operating frequency of said apparatus, said device having a pair of electrically parallel rods, a pair of cylindrical conductive shields for said rods,.the in-- side diameters of said shields being substantially three and one half times the outside diameters of said rods, an adjustable shorting member contacting at points'longitudinally of said rods and means formoving said shorting member into suitable position, for providing a path of low impedance through said parallel rods for stray currents of any frequency lower than the operating frequency of said radio apparatus,

3. A device in accordance withclaim 2 characterized in that the last said means comprises a rotatable driving screw, '9. non-rotatable internally threaded portion of said shorting member,

and manually operable gearing connected with said driving screw.

4. A device in accordance with claim 2 characterized in that each of said parallel rods is substantially inclosed by a metallic cylinderhaving means including an insulating member at least at one end of each cylinder for maintaining the rod and its cylinder coaxial and for insulating portions of each rod between theadjacently disposed conductors and the shorting member.

JOHN EVANS. 

